1. Field of the Invention
This invention pertains to the general field of interferometry and apparatus for testing surfaces. In particular, it provides a novel approach for automatically identifying regions of interest of magnetic heads of computer disk drives, such as air-bearing surfaces, for interferometric-measurement analysis.
2. Description of the Prior Art
The magnetic-head slider of a magnetic disk system operates by floating in very close proximity over the surface of the magnetic disk, thereby accurately reading and writing data thereon. While the slider is floating disposed substantially in parallel over the disk during operation, it must be able to adjust its attitude to conform to magnetic-disk surface imperfections and dynamic displacements, such as surface vibrations generated by the rotating movement. Therefore, the flatness of the slider""s surface facing the disk and the shape and torsional characteristics of the suspension supporting it are critical to the proper functioning of the apparatus and must be maintained within prescribed design specifications to prevent contact with the disk surface and avoid the disabling consequences that normally result from such events.
Accordingly, magnetic heads are tested for quality control by interferometric profiling techniques during manufacture. Since magnetic-head sliders consist of a multilayer structure, it is usually desirable to distinguish between its various layers for testing purposes. For example, magnetic heads include an outer air-bearing surface (first level ABS) designed to provide floating over the surface of the disk, a shallower intermediate region, and a deeper cavity region embedded within the slider. The geometry of all regions varies greatly with different manufacture designs in a continuous effort to improve the sliders"" performance at ever increasing speeds and degrees of miniaturization. Typically, the exact geometry and manufacture tolerances of the ABS region are critical. Therefore, the automated interferometric equipment performing quality-control testing needs to be able to identify and measure the ABS region rapidly and effectively.
Typically, the ABS region of a magnetic head consists of multiple islands separated by intermediate and cavity regions. Prior-art identification techniques use a template approach, where a specific geometry representing the expected configuration of the region of interest is compared to the shape of measured islands of modulation or height data to identify the ABS regions of interest. The approach works very well when the slider being tested is adequately aligned with the template because a substantial overlap will necessarily occur. In practice, though, during the process of manufacture multiple sliders are placed in arrays in plastic trays that serve as the sample stage for interferometric measurement. Each magnetic head is loosely contained within a discrete compartment or well in the tray; therefore, the heads are often misaligned with respect to an optimal position represented by a desirable reference line aligned with the template within the field of view of the interferometric objective.
For example, FIG. 1 illustrates a magnetic head 10 wherein numerals 12,14,16 represent ABS regions according to a given magnetic-head design. Intermediate shallower regions 18,20,22 and a cavity 24 are also illustrated. Assume, for instance, that the head is positioned in the compartment or well 26 of a tray T at an angle a with respect to a reference baseline 28, as illustrated in FIG. 2 in exaggerated condition for the purpose of illustration (note that the baseline 28 is shown to correspond to the bottom edge of the compartment 26 for simplicity, but any other line could be used as well). Assume also that an electronic template 30 with patterns or islands 32,34,36 designed to match the ABS regions 12,14,16 for identification, as shown in FIG. 3, is aligned with the baseline 28 of the tray containing the magnetic head 10 within the field of view of the testing interferometer. It is understood that the initial placement of the template would normally be carried out over the slider""s expected position within the well 26 under optimal circumstances, so as to automatically overlay the slider. As illustrated in FIG. 4, though, in the example at hand, where the slider is shifted with respect to its optimal position, a superimposition of the template over the expected position of the magnetic head would produce a mismatch that would greatly hinder the process of identification of the regions of interest using prior-art techniques. As shown, the pattern 32 in the template 30 would be completely off its intended target, region 12, and the patterns 34,36 would only partially overlay the regions 14,15.
To correct for this misalignment, prior-art template systems have used alignment procedures based on matching predetermined landmarks in the template with regions of data in the sample. For example, if two islands in the template (e.g., 34 and 36 in template 30) are designated for identification of corresponding uniform-height ABS regions in the sample (14 and 16 in slider 10), various regions of measured data in the magnetic head are tested for shapes that match the islands, such as by selecting areas of uniform data having a size and shape approximately equal to the template islands (such as the pixels corresponding to regions 14 and 16 in FIG. 2). When roughly matching regions are identified, a significant marker (such as the center of gravity) is calculated for each region and aligned with the corresponding marker in the template islands. As a result of the procedure, the magnetic head is deemed aligned with the template and the data overlapped by the template islands are used for the interferometric analysis of the regions of interest.
A significant problem with these prior-art procedures is the correct identification of the true regions of interest. If the magnetic head is sufficiently displaced from its ideal position in the tray (i.e., the position overlayed by the initial placement of the template), as illustrated in FIG. 4, the matching algorithm may not be able to identify any matching regions, thereby disrupting the automatic implementation of the test procedure.
Accordingly, it would be very desirable to have a simpler and more practical method and apparatus for identifying the regions of interest in a sample to be tested by interferometric means. This invention provides such a method and apparatus on the basis of a distinct and identifiable geometry of the tested magnetic-head slider.
It is therefore an objective of this invention to provide an automatic procedure for identifying regions of interest in a test sample, so that interferometric testing can be limited to such regions to provide a faster and more efficient testing capability.
Another objective of the invention is a procedure for aligning a sample surface with a template without relying on the recognition of landmarks in the surface.
Another goal of the invention is a method and apparatus that do not require matching of data collected from the test surface with corresponding patterns in a selected template.
A further objective of the invention is that the testing procedure minimize manipulation of the test sample. Another goal of the invention is its general application to instrumentation that requires alignment of the sample to a predetermined template, or vice versa, for the purpose of selectively testing only portions of the sample""s surface.
Finally, another goal is the implementation of the above mentioned objectives in a commercially viable system that maximizes the utilization of existing technology.
In accordance with these and other objectives, the preferred embodiment of the method of this invention consists of providing an electronic template representing the topography of the magnetic head being tested and delineating distinct patterns corresponding to particular regions of interest, such as ABS surfaces, so that analysis of the sample""s surface can be limited to those regions. According to one aspect of the invention, the outline of the slider positioned within the field of view of an interferometric microscope is identified by measuring the modulation of incident light at each pixel inside and outside the contour of the sample using the loci of perceivable fringe contrast as the criterion for establishing the location of such edges. Once the outline of the slider within the plastic tray is so established, the position of the slider is shifted within the field of view coordinates to match the template, thereby automatically achieving a precise alignment of the template with the boundary of the slider. In turn, the template patterns will also become aligned with the regions of interest in the magnetic head and interferometric analysis can be limited to data corresponding to the pixels contained within those patterns.
As a result of the technique of the invention, the regions of interest in a magnetic head are quickly identified and tested, avoiding the unnecessary steps of analyzing the remaining regions of the sample. According to another aspect of the invention, curve-fitting schemes can be used to facilitate and expedite the process of identifying the contour of the slider sample. In the case of conventional magnetic-head sliders, which are substantially rectangular in plan view, straight-line fitting between a few data points along two adjacent edges is sufficient to find the location of the edges and the corner between them, which in turn can be used is straightforward manner to align the template with the slider. Obviously, the minimum such number of data points is four, two for each identified edge of the slider (three points suffice if the angle between the edges is known).
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiments and particularly pointed out in the claims. However, such drawings and description disclose only some of the various ways in which the invention may be practiced.